1. Field of the Invention
The present invention relates to a dichroic dye composition and a guest-host liquid-crystal composition used for a liquid-crystal element in a display element, a dimmer element, or the like, and to a liquid-crystal element using such a guest-host liquid-crystal composition.
2. Description of the Related Art
Liquid-crystal elements supporting liquid-crystal compositions between electrodes have been widely used for display elements such as notebook computers and personal data assistance (PDA), dimmer elements such as camera viewfinders capable of switching views, and the like, because of the space-saving and light-weight characteristics. Its application range has been widely spread year after year.
Most of currently-used liquid-crystal elements are mainly of twisted nematic (TN) type or super-twisted nematic (STN) type. However, because these elements use polarizers, there is disadvantage that the light-use efficiency is low. On the other hand, liquid-crystal elements applying the guest-host liquid-crystal compositions with dichroic dyes utilize the absorption anisotropy of the dichroic dyes so that the light-use efficiency is high without the polarizer. Thus, particularly in the case of reflective display elements not provided with backlights, luminous display can be achieved.
The dichroic dye indicates various absorption characteristics according to its dye structure; however, the dichroic dye mostly absorbs specific wavelengths such as red, blue, and yellow so that it is difficult to show black using a single dye. For this reason, three or more of a plurality of dichroic dyes are usually mixed in order to show black. Generally, the wavelength range of visible lights is thought to be from 380 nm to 780 nm, and it is recognized as “black”, if the light absorption is constant in this range. However, in consideration of the standardized visual sensitivity curve of dark adaptation with regard to human visual sensitivity, the absorption is desirably constant in the wavelength range from 400 nm to 650 nm.
For the dichroic dye used for the guest-host display elements, various characteristics are also requested including high order parameters, high solubility with respect to the liquid-crystal, and the like. In the guest-host liquid-crystal compositions, the order parameters and the solubility with respect to the dichroic dye liquid-crystal largely affect display quality of the guest-host display elements in terms of contrast, color reproductivity, and the like. Among them, although the contrast can be improved by increasing concentration of the dichroic dye, transmissivity (or reflectivity) of decolorized colors is decreased correspondingly and the luminous display as a characteristic of the guest-host display elements is lost. Therefore, the abovementioned two characteristics are requested to be maintained at the same time.
However, because a single dichroic dye has various characteristics in the related art, in mixing system of a plurality of dichroic dyes, it is difficult to decrease the wavelength dispersion of absorbance in the wide wavelength range while satisfying the solubility and dichroism. Particularly, in the case that a host liquid-crystal is a fluorinated liquid-crystal composition, the reduction of the dye solubility is remarkable. Therefore, there has been a difficulty in the development of the guest-host liquid-crystal composition which indicates the high order parameters, with use of the fluorinated liquid-crystal.
As the guest-host liquid-crystal composition having the relatively high order parameters, some have been proposed in Japanese Unexamined Patent Publication Nos. 2000-313881, 2000-17266, and 2000-73064, and Japanese Unexamined Patent Publication No. Hei10-95980. However, in these patent documents, the order parameters in the wavelength range from 400 nm to 500 nm are reduced in comparison with the case of the wavelength range from 600 nm to 650 nm, and the present situation is that the guest-host liquid-crystal composition having the high order parameters in the entire wavelength range from 400 nm to 650 nm has not been provided.